It is occasionally desirable that a fluorinated polymer structure be porous. Such porous structures can be used in diverse applications such as in filtration, for porous diaphragms, and for reducing weight in fluorinated polymer structures. Porosity can increase the available surface area of a fluorinated polymer structure utilized for supporting a catalyst and thus considerably enhance catalyst loading capabilities for a particular fluorinated polymer structure supporting a catalyst where surface effects are of importance.
A variety of techniques are known for forming pores in a fluorinated polymer structure. In one technique, an expandable pore precursor is introduced into the fluorinated polymer. The precursor is then subjected to an environment, usually an elevated temperature, whereby the pore precursor grows substantially in size, forming a pore within the fluorinated polymer. Frequently such precursors then escape from the structure through interlocking pores or are removed in any of a number of well-known suitable or conventional manners.
In another technique, a particulate pore precursor is blended into a fluorinated polymer. The particulate is selected to be of approximately dimensions desired in pores in the completed fluorinated polymer structure. Following completion of the fluorinated polymer structure, the pore precursors are removed using well-known techniques such as chemical leaching and the like. Removal of the pore precursors leaves the fluorinated polymer structure porous.
For some fluorinated polymers, heat activation of a pore precursor may damage or degrade the fluorinated polymer structure. Where the fluorinated polymer is possessed of special physical properties such as ionic exchange functionality, heat activation of a pore precursor can significantly effect those special physical properties.
Particularly, heat activation can adversely effect cationic exchange properties of certain resins utilized frequently for fabrication of cationic exchange membranes. One copolymeric ion exchange material finding particular acceptance has been fluorocarbon vinyl ether copolymers known generally as perfluorocarbons and marketed by E. I. duPont under the name Nafion.RTM..
These so-called perfluorocarbons are generally copolymers of two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.
The second monomer is selected from a group of monomers containing an SO.sub.2 F or sulfonyl fluoride group. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising one to eight carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbon atom adjacent the --SO.sub.2 F, particularly where the functional group exists as the --(--SO.sub.2 NH)mQ form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups. Perfluorocarbon copolymers containing perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer have found particular acceptance in Cl.sub.2 cells.
Presently, perfluorocarbon membranes, for example, are generally fabricated by forming a thin membrane-like sheet under heat and pressure from one of the intermediate copolymers previously described. The ionic exchange capability of the copolymeric membrane is then activated by saponification with a suitable or conventional compound such as a strong caustic. Generally, such membranes are between 0.5 mil and 150 mil in thickness. Reinforced perfluorocarbon membranes have been fabricated, for example, as shown in U.S. Pat. No. 3,925,135.
These membranes have been utilized in electrochemical cells. Notwithstanding the use of such membrane separators, a remaining electrical power inefficiency in many batteries, fuel cells and electrochemical cells has been associated with a voltage drop between the cell anode and cathode attributable to passage of the electrical current through one or more electrolytes separating these electrodes remotely positioned on opposite sides of the cell separator.
Recent proposals have physically sandwiched a perfluorocarbon membrane between an anode-cathode pair. The membrane in such sandwich cell construction functions as an electrolyte between the anode-cathode pair, and the term solid polymer electrolyte (SPE) cell has come to be associated with such cells, the membrane being a solid polymer electrolyte. In some of these SPE proposals, one or more of the electrodes has been a composite of a fluororesin polymer such as Teflon.RTM., E. I. duPont polytetrafluoroethylene (PTFE), with a finely divided electrocatalytic anode material or a finely divided cathode material. In others, the SPE is sandwiched between two reticulate electrodes. Typical sandwich SPE cells are described in U.S. Pat. Nos. 4,144,301; 4,057,479; 4,056,452 and 4,039,409. Composite electrode SPE cells are described in U.S. Pat. Nos. 3,297,484; 4,212,714 and 4,214,958 and in Great Britain patent application Nos. 2,009,788A; 2,009,792A and 2,009,795A.
Use of the composite electrodes can significantly enhance cell power efficiency. However, drawbacks associated with present composite electrode configurations have complicated realization of full efficiency benefits. Composite electrodes generally are formed from blends of particulate PTFE TEFLON and a metal particulate or particulate electrocatalytic compound. The PTFE blend is generally sintered into a decal-like patch that is then applied to a perfluorocarbon membrane. Heat and pressure are applied to the decal and membrane to obtain coadherence between them. A heating process generating heat sufficient to soften the PTFE for adherence to the sheet can present a risk of heat damage to the membrane.
These PTFE TEFLON based composites demonstrate significant hydrophobic properties that can inhibit the rate of transfer of cell chemistry through the composite to and from the electrically active component of the composite. Therefore, TEFLON content of such electrodes must be limited. Formation of a porous composite has been proposed to ameliorate the generally hydrophobic nature of the PTFE composite electrodes, but simple porosity has not been sufficient to provide results potentially available when using a hydrophyllic polymer such as NAFION in constructing the composite electrode.
To date efforts to utilize a hydrophyllic polymer such as NAFION have been largely discouraged by difficulty in forming a commercially acceptable composite electrode utilizing perfluorocarbon copolymer. While presently composites are formed by sintering particles of PTFE TEFLON until the particles coadhere, it has been found that similar sintering of perfluorocarbon copolymer can significantly dilute the desirable performance characteristics of perfluorocarbon copolymer in resulting composite electrodes.
For even hydrophyllic materials such as NAFION, a porous structure can considerably enhance contact between an electrocatalytically active component distributed throughout the structure and any reactants. Possible damage to ionic exchange functionality of a NAFION structure from the use of a heat activated pore precursor is likely at elevated temperatures. Other conventional pore forming techniques such as incorporation of a pore precursor into the fluorinated polymer structure followed by subsequent removal of the precursor often introduces additional processing steps making fabrication of a desired structure undesirably complicated.